Production of colour prints



Aug. 2s, 1962 D. M. NEALE PRODUCTION OF COLOUR PRINTS Filed Jan. 18, 1961 United States Patent @face 3,051,069 Patented Aug. 28, 1962V Claims priority, application Greaty Britain Jan. 2,5, 1960 1 Claim. (Cl. 95-73) This invention relates to the printing of multi-colour photographic records and particularly to the vprinting of colour negatives' to produce positives to be viewed by reflection.

When a `colour negative is printed on to multi-colour photographic material (commonly paper), it is important that the integrated exposures to three bands of the printing spectrum shall be correctly proportioned. lf these three exposures are not correctly proportioned, the print, after processing, will exhibit a colour cast, Whereas experience teaches that for the majority of cases the optimum condition is that where the integrated colour of the print is grey. The optimum relative propor-tion of the exposures depends upon thek nature of the negative, the light source, the relative'sensitivities of the three emulsion layers in the print material and the processing conditions. Known methods of monitoring photoelectrically the quality of light transmitted -by the negative simplify the accommodation of the printing process to variations in colour quality of the negative `or light source, but the adjustment -of the apparatus to suit variations in paper sensitivity, such as may occur between one batch and another, has remained cumbersome or uncertain. Similarly, adjustment of the monitoring process to take account of colour deviations due to variations in processing conditions has also been a cumbersome process hitherto and one involving the production and assessment of numerous test strips.

The present invention relates to an improved method of producing multi-colour photographic prints from multicolour negatives whereby it may be readily established which quality of light is required to expose a print so that, after processing, the integrated Acolour of the print is grey.

The invention comprises a method for printing multicolour photographic records on to multi-colour lightsensitive material wherein some of the printing light is caused to fall on colour selective filters behind which are one or more photocells so that the colour quality of said light may be assessed by comparing the electrical outputs from the combinations of said filters and photocells, making and processing a trial print, placing said processed trial print in the path of light reaching said photocells so that light reaching said photocells is modified by the colour of said processed trial print, 1re-adjusting the colour of printing light in accordance with said modification of light reaching said photocells and using said re-adjusted printing light to expose further prints.

The theoretical foundation for the method will now be described with reference to a negative-positive process.

Suppose that there is placed in the gate of an enlarger a negative which should give a print as a uniform grey, but that the colour quality of light reaching the print material is excessively blue by 0.1 log intensity units. If the negative-positive process is one in which blue light is used to expose that emulsion layer i-n the print which subsequently develops to a yellow image component, then a print made by exposure to this quality of light will, after processing, appear as a yellow-brown, i.e. as a grey overlaid With yellow. The departure from grey will be 0.13/c reflection density units where 'yc is the colour contrast of the print material to reflected light.

Now the relation between reflection density Dr and transmission density D, of a paper print (both measured 2 above` base density) is of the general character indicated by FIG. l of the accompanying drawings. f

Over'the greater part of the density range the value of, dDr/dD, approximates to 2.() Thus the colour of the processed print discussed above, measured by transmissi-on, will be more yellow than the colour of processed, unexposed print material, also measured by transmission, by 0.1fyc/2-0 density units. Typical colour print materials 'have values of y., approximating to 2.0 Thus in the above example the colour of the processed print will be approximately 0.1 -density units more yellow than thea processed, unexposed print material when the density Aof each is measured by transmitted light. This colour differential is, of course, that required to correct the balance( of a printing light which -is too blue by 0.1 log intensity units.

`A preferred method of applying the invention to the production of prints by a negative-positive process will now` be described with reference to FIG. 2 of the` drawings.

In FIG. 2, light from a source 1 passes through an adjustable colour modulator of known type (eg. movable colour filters `or colour'wedges) shown diagrammatically at 2, through a diffuser 3, negative 4, and enlarging lens 5, to strike paper 6 on the easel 7. For the establishment of colour balance, i.e. the colour quality of printing light required in order that a given batch of printing paper shall, rafter processing, be grey, the negative 4 should preferably resemble a typical colour negative of a uniform grey area. If the approximate colour quality required is known, the colour modulator 2 should be set to produce this colour of light at the easel before exposing a trial print 6. The exposure time is perferably chosen so that the reilection density of the print is, after processing, near the middle Iof the density range of the print material.

A reflector 8 is placed in the light path to divert some of the printing light on to the `emulsion side of a piece of unexposed, iixed-out print material 9. Light transmitted by the print material 9 passes through a diffuser 10 to enter an integrating box 11. Red, green and blue filters, 12, 13 and 14 respectively, allow only red, green or blue light to fall on each Iof three photocells, 15, 16 and 17 respectively. The photocells are connected to a colour monitoring device 18 of known type.

Light density of the quality used to expose the trial print on paper 6 is allowed to pass through the unexposed, fixed-out print material 9 and `diffuser 10, and the colour of the resultant integrated light is assessed by the filters 12, 13, 14, photocells 15, 16, 17, and monitoring device 18. After the trial print has been processed, this assessment is checked to re-establish the same colour quality of printing light. If necessary, the colour modulator 2 may be adjusted to restore the printing light to its original colour quality. The unexposed, fixed-out print materal 9 is now replaced by the processed trial print on paper 6 the emulsion of which again faces the incident light. The colour of the light entering the integrating box is now assessed again and this new Value will hereafter be referred to as the corrected assessment. The trial print 6 is now removed and the unexposed, fixed-out print material 9 is replaced 4in the position it originally occupied. The colour modulator 2 is now operated until that colour of printing light is obtained which provides on the monitoring equipment 18 a colour assessment coinciding with the corrected assessment. The colour quality of the printing light is now that which, when used to expose material similar to that of the trial print on the paper 6 will produce a print which, after processing in the same Way as the trial print, is substantially grey.

The invention may also be applied to reversal or other positive-positive processes. The procedure in this case follows that already described with reference to FIG. 2 so far as the processing of the trial print on paper 6 is concerned. Thereafter, the unexposed fixed-out material 9 is replaced by the processed trial print on paper 6 and the colour modulator 2 is adjusted to produce on the monitoring equipment 18 a colour assessment coinciding with the original assessment. The colour quality of printing light is now that which, when used to expose material similar to that of the trial print on paper 6 will produce `a print which, after processing in the same Way as the trial print on paper 6, is substantially grey.

In some print materials y@ considerably exceeds the maximum Value of dDr/dDt. It will be readily understood that in such a case there is a tendency for overcorrection to arise. Thus if the printing light was initially 0.1 log intensity units too blue, the procedure may indicate a correction by more than 0.1 density units towards yellow.

A similar ditl'iculty arises in print materials intended to be viewed =by transmitted light. The colour contrast of such materials is often made about 2.0 to transmitted light rather than reected light. There is thus again a tendency to over-correction.

Such over-correction may be reduced or eliminated by electrically, ioptically or mechanically arranging that only a proportion of the indicated correction is applied to subsequent printing.

What I claim is:

A method for printing multi-colour photographic records on to multi-colour light-sensitive material with light having red, green and blue components which comprises causing some of the printing light to fall on colour selective lters behind which is at least one photocell, assessing the colour quality of said light by comparing the electrical outputs from each combination of filter and photocell, making and processing a trial print, placing said processed trial print in the path of light reaching said combinations of lter and pliotocell so that lightreaching said combinations is modified by the colour of said processed trial print, re-adjusting the colour `of printing light in accordance With said modification of light reaching said combinations, said modification of the colour `of printing light being made to modify the relative proportions of the red, green and blue components of printing light by amounts substantially proportional to the modication of :the relative proportions of the red, green and blue components of light transmitted by the said trial print and said modioation `of colour of printing light being in that sense necessary to reduce the colour cast associated with the trial print, and using said -re-adjusted printing light to expose further prints.

No references cited. 

